This invention relates to vehicle wheel alignment systems, and in particular to an sensors in a vehicle wheel alignment system which include an active pixel array detector such as a charge coupled device (CCD) array.
Proper alignment of wheels in a vehicle is imposer for proper handling of the vehicle and proper tire wear. The alignment of a vehicle's wheels is performed primarily by adjusting camber, caster, steering axis inclination (SAI) and toe. Other suspension problems may be detected by examining vehicle ride height (a measure of vertical vehicle body position with respect to some reference such as the ground or a vehicle wheel) and wheel offset distances (a measure of relative horizontal vehicle body position with respect to the vehicle wheels). Some of these alignment measurements may also require determination of vehicle distances such as wheelbase and track width. Unfortunately, many of these measurements are not easily performed using conventional alignment systems.
It is known that alignment angles may be measured by placing sensor heads on each wheel of the vehicle to form pairs which extend across the front and/or rear of the vehicle and along each side of the vehicle. Each sensor head typically has an emitter and a receiver. In the prior art systems a sensor head emits a signal which is transmitted to the receiver of the other sensor head of that pair. The receiver conveys this signal into a value which is indicative of the measured angle. The signal presently used in these sensor heads is an electromagnetic signal in the visual or infrared range (hereinafter, referred to as light). The light impinges upon a sensing device in the receiver whose output is representative of the measured angle.
Currently, photodiodes, as set forth in U.S. Pat. No. 4,302,104, which is incorporated herein by reference, and linear array type charge coupled devices (CCDs), as set forth in U.S. Pat. No. 5,018,853, are used as the receiver. Each of these devices has inherent limiting factors which affect their suitability for use as detectors in a vehicle wheel alignment system.
The photodiode array has a number of large active areas arranged in a linear array. The incident angle of the light beam is determined by taking the ratio of signals on two adjacent elements of the array. Because the elements are large, the aperture opening typically has a correspondingly large area. It should be understood that the size and placement of the aperture in these systems involves a compromise. If the photodiode elements are large and the slit or aperture is close to it, the system will be able to measure a wide angular range, but the resolution will be lowered. On the other hand, if the diode is small and the slit is far away, the resolution will be high but the angular range will be small. Under certain conditions, diffuse and/or reflected beams can enter the sensor and bias the angular readings. This occurs when a reflected beam and the directly radiated beam are co-incident on the array, but their energies are centered at different points. The reflection cannot be identified (and thereby rejected) by the electronics because the large photodiode uses all the energy that is incident upon it (direct and reflected) to produce a signal. Because the reflection cannot be eliminated, a false reading, and hence a wrong angle, will be produced.
The CCD array incorporates many more active elements than the diode array, and those elements are much smaller than the diode array elements. As a result, spatial resolution with a CCD array is greatly enhanced and reflected beams that are co-incident in time but not position with the main beam can be identified and separated before the angle is determined. Problems can still be caused, however, by reflected signals if they are large in amplitude and impinge close to the main beam.
Although alignment sensors are quite accurate, it should be understood that there is some variation in response from sensor to sensor caused in part by variations in mechanical construction. It would be desirable to compensate for this variation by automatically range calibrating the sensors initially so that subsequent measurements would be uniform. In addition, it would be desirable to reduce the cycle time between measurements for current sensors. Many CCD arrays, for example, have an undesirably long minimum cycle time which is dependent upon the read out cycle time of the sensor. In addition all current sensors are believed to be affected by ambient light sources which interfere with the functioning of the sensors. To compensate for this effect, some sensors expose the image array twice: once with the light source on to measure the angle in question, and once with the light source off to determine the ambient light levels. The difference of the two readings shows only the desired light source. This procedure, however, requires two exposures, which further degrades the total cycle time of the sensor.
It has been found that the performance of vehicle alignment sensors can be improved by using a pair of light sources for each detector, but this requires determining the actual angle from the pair of resulting images on the sensor. Since the sensors are used in pairs, it is also desirable to synchronize the operation of the sensors so that the light from both emitters of a sensor pair is not radiated at the same time.
In order to define the measured angle accurately, the prior art devices have used various means such as narrow slits, or long exposure times to define the image. A narrow slit interposed between the radiation source and the detector array provides a satisfactory image, but it significantly reduces the amount of energy reaching the detector array. A very high intensity radiation source (or the aforementioned long exposure times) can be used to compensate for the effect of the narrow slit, but that is also undesirable. Moreover, even when adequate radiation is available and the image is sharp, the relationship between the position of the image and the angle being measured is not linear except for small angular variations from normal incidence. Off-axis angles of incidence may be determined accurately from the detector output only by mathematical manipulation of the detector output, which slows down the response of the system and also complicates the range calibration of the system.
Conventional alignment systems are not optimally suited to make various desirable alignment measurements. One example is measurement of toe out on turns (hereinafter "TOOT"). In order to minimize tire wear and enhance directional stability, the front wheels of a vehicle turn at slightly different angles when negotiating a turn. The ideal angles are determined by the vehicle manufacturer and are largely dependent on wheelbase, track width and the radius of the turn. Many vehicles have these angles specified at twenty (20) degrees of turn. Some manufacturers specify setting the inside wheel at twenty (20) degrees and measuring the toe of the outside wheel, while others specify the outside wheel at twenty (20) degrees and measure the toe of the inside wheel. In either case there is a nominal angle that is to be achieved by each wheel. Most available aligners do not have the angular range necessary to measure twenty (20) degrees of turn by purely electro-optical means. Various mechanical expedients are used instead. For example, the assignee of the present application makes a kit for this purpose consisting of angle encoders and elastic strings. A string is connected between the encoders in the front and rear sensors and the total angle of turn is computed by subtracting the rear encoder reading from the front encoder reading. While this method works, it has relatively low resolution and the additional disadvantage of the physical attachment of the strings. Alternatively, the steered angle may be measured by an angle sensing turn plate, a completely separate piece of equipment. The turn plate sits under the desired front wheel and pivots with the wheel as it turns. Such systems are not accurate enough if there is any slippage between the tire and the turn plate (which is often the case if the turn plate is not well centered on the tire).
It would be desirable to measure the steered angle of the proper front wheel using electro-optical means, but that approach also has drawbacks. These arise from the fact that such a conventional system would require the front and rear sensor on the proper side of the vehicle to "see" each other simultaneously over an angular range of at least twenty (20) degrees. This is impractical because many currently available emitters do not have sufficient radiation intensity over such a wide emission angle. In addition, if the emitter beam is not confined to an approximately five (5) degree cone, there may be undesirable reflections occurring due to the proximity of vehicle body and chassis parts. If the reflections are severe, they could render the system inoperative. With the five (5) degree emitter cone restriction, the rear sensor in a conventional system will loose sight of the front sensor emitters as soon as the front wheel turn angle or steered angle exceeds five (5) degrees. However, the front sensor can see the rear emitter(s) and calculate angles all the way to twenty (20) degrees of turn (since the detector on the front wheel remains in the five degree cone of light from the rear emitter throughout). Unfortunately, using only the angle from the front sensor results in erroneous readings because the front sensor itself is swinging through space at the end of an are which is centered on the steering axis. The translation due to the swing is interpreted as an additional angle by the sensor.